The present disclosure is related to a motor driving technology.
FIG. 1 shows a model diagram of a three-phase brushless direct current (DC) motor 500. The motor 500 includes: respective stator cores 502 for U-phase, V-phase and W-phase; a stator including stator coils (hereafter referred to coils) LU, LV and LW reeled in an activator core; and a magnetic rotor 504 having a permanent magnet.
The coils LU, LV and LW are in sequence powered (also called phase switch) by a driving circuit (not shown), such that a winding field 506 produced by the stator rotates. The winding field is applied on the magnetic rotor 504, so as to rotate the magnetic rotor 504.
It is well known that when the torque of the motor at the winding field 506 and the magnetic rotor 504 substantially have the relationship in position as shown in FIG. 1, more specifically, when winding field 506 is orthogonal to the magnetic rotor 504, the torque of the motor is the greatest.
If an equivalent circuit of an inductor L and a resistor R is accounted for a motor, formula (1) is established between a coil current i(t), a voltage V (also called applied voltage, driving voltage) between terminals, and an induced voltage e (also called counter electromotive force).V−e=R·i+L·di/dt=(R+jωL)·i  (1)
Formula (2) is obtained by varying formula (1).i=(V−e)/(R+jωL)=(V−e)·(R−jωL)/(ω2L2+R2)  (2)
In other words, the phase of the coil current i is varied according to a composite vector (V−e) of the driving voltage V and the induced voltage e and the resistance of the motor (jωL+R). Specifically, the phase of the coil current has a delay θ=arc tan(ωL/R) relatively to the composite vector (V−e) of the driving voltage V and the induced voltage e.
FIG. 2(a) shows the relationship among the driving voltage, the induced voltage, and the current flowing through the coil of a predetermined phase. The left drawing of FIG. 2(a) shows the status that the driving voltage V is generated with the same phase as the induced voltage e. In the status, the phase of the coil current i delays relatively to the induced voltage e.
The right drawing of FIG. 2(a) shows the status that the driving voltage V leads the induced voltage e. In the status, the coil current i and the induced voltage e have the same phase, so as to obtain a high torque.
FIG. 2(b) is a vector diagram showing the phase relationship of the right drawing of FIG. 2(a). In order to make the induced voltage e and the coil current i have the same phase, the composite vector (V−e) of the induced voltage e and the driving voltage V shall have a θ delay relatively to the coil current i, and the amplitude and the phase of the driving voltage V are regulated in such way.
Herein, the induced voltage e is proportional to the turning number ω of the motor, and the phase leading angle θ affected by the resistance of the motor is varied instantaneously according to the turning number ω of the motor. Therefore, in order to obtain the maximal torque, the amplitude and the phase of the driving voltage V have to be instantaneously varied according to the turning number ω of the motor. It is also called a leading angle control since the phase of the driving voltage V leads the phase of the induced voltage e.